1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying apparatus, a laser beam printer and the like for forming a toner image corresponding to an original image or inputted image signal on a recording material in a transferring manner or a direct manner, and more particularly, it relates to a fixing apparatus for fixing a non-fixed toner image formed on a recording material used in such an image forming apparatus to the recording material.
2. Related Background Art
For example, in image forming apparatuses such as copying machines and printers utilizing transfer-electrophotographic process, a photosensitive member (image bearing member) uniformly charged by a charger is image-exposed by illuminating light corresponding to an original image or an image signal inputted from a computer to thereby form an electrostatic latent image on a surface of the photosensitive, and then, the electrostatic latent image is developed by a developing means as a toner image, and the toner image is transferred onto a transfer material (recording material) by a transfer means, and the toner image on the transfer material is fixed to the transfer material by a fixing means to obtain an imaged product (copy, print).
As one of fixing devices for fixing the non-fixed toner image formed on the transfer material to the transfer material as a permanent fixed image, there is a fixing device of on-demand film fixing type.
In this fixing device of on-demand film fixing type, a heat-resisting fixing film is slidingly contacted with a heating member and a pressurizing member is urged against the heating member with the interposition of the fixing film to form a nip so that the non-toner image is thermally fixed to the transfer material by thermal energy from the heating member via the fixing film while the transfer material (bearing the non-fixed toner image thereon) is being introduced between the fixing film and the pressurizing member at the nip and is being conveyed together with the fixing film.
An example of such a fixing device of film fixing type is shown in FIG. 7. A ceramic heater (heating member) 120 is secured to and supported by a lower part of a heat-resisting and heat-insulating stay (heater supporting member) 21 having rigidity. An endless fixing film 25 formed by coating material having good mold releasing property on high heat-resisting material and mounted on a drive roller 26 and a driven roller 27 (which are disposed substantially in parallel with the heater 120) is slidingly contacted with the heater 120.
A pressure roller (rotatable pressurizing member) 128 is urged against a lower surface of the heater 120 with the interposition of the fixing film 25. The pressure roller 128 has an elastic rubber (for example, silicone rubber) layer having good mold releasing property and cooperates with the heater 120 to form a fixing nip N therebetween.
When the drive roller 26 is rotated by a drive motor (rotation driving means) M in a direction shown by the arrow, the fixing film subjected to predetermined tension from the driven roller 27 (also acting as a tension roller) is rotated at a predetermined peripheral speed in a direction shown by the arrow a, and the pressure roller 128 is rotatingly driven in an anti-clockwise direction by movement of the fixing film 25.
In a condition that the fixing film 25 is rotatingly driven and the heater 120 is heated and heat-adjusted to a predetermined temperature, a transfer material P (on which a non-fixed toner image T was formed) outputted from an image forming means (not shown) is introduced between the fixing film 25 and the pressure roller 128 at the fixing nip N with the imaged surface facing the fixing film 25, so that the transfer material P is conveyed through the fixing nip N together with the fixing film 25 with the imaged surface of the transfer material closely contacted with the outer surface of the fixing film 25; meanwhile, the non-fixed toner image T is thermally fixed to the surface of the transfer material P by thermal energy from the heater 120 via the fixing film 25.
In such a fixing device of film fixing type, electric power can be saved and a waiting time can be shortened (i.e., quick starting ability can be achieved) on demand, since heat capacities of the heater (heating member) 120 and the fixing film 25 for transmitting the thermal energy from the heater 120 to the transfer material P can be reduced.
However, the fixing device of film fixing type has the following disadvantages.
That is to say, when the non-fixed toner image T on the transfer material P is being passed through the fixing nip N, a small amount of toner is transferred from the transfer material P to the outer surface of the fixing film 25 (offset phenomenon).
A part of the toner offset to the outer surface of the fixing film 25 is transferred onto the same transfer material P again after the fixing film 25 is rotated by one revolution or is remaining on the fixing film 25 and then transferred onto a next transfer material P.
Since the amount of toner transferred to the transfer material is very small, there is no practical problem.
However, a part of the toner offset to the outer surface of the fixing film 25 is also transferred onto the surface of the pressure roller 128. When the toner is once adhered to the pressure roller 128, the mold releasing ability of the portion of the pressure roller 128 to which the toner is adhered is considerably worsened, so that the toner is apt to be adhered to such a portion. Consequently, a land-shaped thin toner layer is formed on the surface of the pressure roller 128.
An adhering force of the thin toner layer to the pressure roller 128 is weak when a surface temperature of the pressure roller 128 is low (particularly, when the surface temperature is lower than a glass transition temperature).
In the fixing apparatus of this kind, when the apparatus is in a waiting condition, the heater (heating member) 120 is not energized. That is to say, the heater 120 is not heated. Accordingly, the temperature of the pressure roller 128 is normally maintained to substantially a room temperature (i.e., below the glass transition temperature).
In such a condition, when the fixing operation is started, the temperature of the surface of the fixing film 25 is quickly increased at the fixing nip N. Before the transfer material P reaches the fixing nip N, the surface of the thin toner layer adhered to the surface of the pressure roller 128 is softened and is closely contacted with the fixing film 25. As a result, a part of the thin toner layer on the pressure roller 128 is separated from the pressure roller 128 and is transferred onto the fixing film 25. The toner transferred to the fixing film 25 then reaches the fixing nip N, where the toner is successively transferred to the transfer material being passed through the fixing nip. As a result, the surface of the transfer material is contaminated in spots.
In order to prevent the spotted contamination on the transfer material P due to the toner contamination of the pressure roller 128, as shown by the two-dot and chain line in FIG. 7, there has been proposed a technique in which a roller 133 formed from metal such as aluminum having good heat conductivity and high surface energy is contacted with the pressure roller 128 and is rotatingly driven by rotation of the pressure roller 128 so that the toner is transferred from the pressure roller 128 to the roller 133 to collect the toner.
However, in the normal use, the toner (on the pressure roller 128) contacted with the roller 133 is not thermally deformed, since a surface temperature of the roller 133 is not increased up to the glass transition temperature of the toner to thereby worsen the collecting ability of the toner from the pressure roller 128 to the roller 133.
Further, although the roller 133 is rotatingly driven by the rotation of the pressure roller 128, the roller 133 is also rotated only during the fixing operation, since the pressure roller 128 is normally rotated only during the fixing operation, so that the cleaning of the pressure roller 128 becomes insufficient.